Several types of botanical products containing isoflavones are being marked as an alternative to estrogen replacement therapy to prevent osteoporosis in postmenopausal women. The efficacy of these products is unknown. Estrogen suppresses bone resorption and may increase calcium absorption. Similar mechanisms of action are not understand for isoflavones partially because of methodological limitations in measuring bone resorption. The specific aims of this proposal are: 1. To identify the effective dose of soy protein isolate enriched with isoflavone concentrates on suppressing bone resorption, and 2. To establish the effective dose of isoflavone concentrates with different isoflavone profiles from soy cotyledon and germ, red clover leaf and flow extract, kudzu, and individual isoflavones on suppressing bone resorption. We hypothesize, based on our preliminary data, that isoflavones will not affect calcium absorption but will suppress bone resorption. We will use 41 Ca, a long- lived isotope, to label the skeleton and monitor perturbations by diet and an Accelerator Mass Spectrometry facility. This innovative approach to study bone resorption is free from noise and is a direct measure of bone resorption in contract to biochemical markers of bone turnover and traditional calcium kinetics, rapid compared to studying changes in bone density, and not invasive as is bone histomorphometry. We will be able to systematically determine the effect of various types and dosages of isoflavone-containing botanical products sequentially in the same post- menopausal women for their effectiveness in suppressing bone resorption by measuring changes in 41 Ca excretion in the urine.